As mentioned in the Mt Nicholson thread, I came across this PB back in 2000 and have never been back since.
It's to the South of the old Mt Nicholson governemnt apartments, now demolished for the new Wheelock development. I remember the PB being located on a flat, slightly elevated piece of ground, with steps leading up to it. I recollect it being fairly exposed, i.e. not dug into the hill. Someone had cultivated a garden in the flat ground beside it.
The heavy door was lying nearby, but other bits of iron work were still in place. From memory I think it was a PB design and not an artillery observation post.